The invention relates to methods for manufacturing hollow thermoplastic objects whereby in a first step, a blank is manufactured by injection prior to obtaining the final container during a method comprising at least one blowing step.
The blank obtained by injection usually has a tubular cylindrical body that is closed at one of its axial extremities and is extended at its other extremity by a neck, also tubular. The neck is usually injected so that it already has its final shape whereas in order to shape the final container, the body of the blank must undergo a rather significant distortion following the blowing operation.
These methods relate in particular to the manufacture of polyethylene terephthalate (PET) containers.
Several variants of these methods make up this family. Based on the method used, the corresponding manufacturing installation comprises one or several thermal conditioning ovens.
In all the methods in question, the blowing of the body of the blank requires that the latter be brought to a temperature that is greater than the glass transition temperature of the material. With this end in view, the blank is subjected to thermal conditioning by having it circulate inside an oven. The oven comprises heating means that are for example made of infrared lamps in front of which the blank is moved using a conveyor element. Advantageously, it is provided that the blank rotate on itself during while being moved in the oven.
The blanks are usually held on the conveyor elements by a gripper device consisting of a mandrel that is engaged inside the blank""s neck and holds the latter in place by tightening it against the inner face of the neck. However, in certain cases, the blank is gripped by the outer face of the neck.
In some of these methods, we seek to increase the temperature hold of the final container""s body by increasing the PET""s crystallinity ratio. This is done by first subjecting the blank to a first blowing phase in order to obtain an intermediate container that in turn is subjected to a heating phase in a thermal treatment oven, a phase during which it shrinks. This shrunken container is then subjected to a second blowing phase that gives it the shape of the final container.
In another method, the neck of the blank, the final container or an intermediate container is what we want to heat in order to increase its crystallinity. Therefore, the installation comprises an adapted thermal treatment oven.
In all cases, we see that the installations for the manufacture of thermoplastic containers can comprise several types of thermal treatment ovens, where these ovens are of course adapted to the thermal treatment that is to be performed. However, all these ovens comprise a conveyor system.
These various systems are thus meant to convey objects that correspond to the various manufacturing phases of the final container. However, they are all conceived to seize this object by the neck, which does not undergo any basic geometric variation, even if certain treatments lead to a change in the structure of the material that constitutes the neck. Also, in the text that follows we will indifferently use the term preform, blank or bottle to designate the object that is to be treated in the oven.
According to a known technique, the conveyor system can for example be comprised of a conveyor link chain that moves continuously on itself.
To seize the preform, a conveyor element comprises a gripper device that moves in an axial direction in relation to the conveyor element and that, when brought from an extreme cleared position to an extreme gripping position, engages in an axial direction on the neck of the blank.
For this to take place, the blank must be brought under the corresponding conveyor element and must be held directly above said element throughout the entire duration of the seizing operation. Yet, during this time, the conveyor element moves continuously, therefore, the blank must also be moved continuously.
With this end in view, the blank is for example supported by a loading wheel that comprises a disk that is driven in rotation around its axis and is fitted, on its periphery, with notches that are more or less semi-circular. An arc of circle guide is arranged around a portion of the disk""s periphery in such a way that a blank can be clasped in a radial direction between a notch of the disk and the guide. The blank then rests by a collar located at the base of its neck on both the disk and the guide, where the neck and the body of the preform extend respectively above and under the level of the disk. Thus, the preform is carried by the loading wheel and, through the rotation of the disk, is driven following a circular trajectory.
The loading wheel is arranged in such a way that the circular trajectory of the preform is at a point that is at a tangent to the direction in which the conveyor system moves. The movements of the wheel and the conveyor elements are synchronized so that, at this point of tangency, the blank is arranged in an axial direction directly over the gripper device of one of the conveyor elements. Thus, at this point, the gripper device can be made to move towards its low position to seize the blank by the neck.
Therefore, we note that, at the point of tangency, the blank still rests on the loading wheel and on the guide, which stops right after this point to allow the blank to follow its new trajectory, which is that of the conveyor element.
In theory, at the loading point level, the blank rests by its collar on the loading wheel and its position is perfectly defined by the notch of the disk in which it is clasped.
However, in practice, it is necessary to provide for operating clearances between the blank, the disk and the guide. Therefore, the blank has a certain range of movement in relation to said two elements. Yet, we noted that this range allowed the blank to vibrate on the loading wheel. This is particularly noticeable in high pace installations in which the speed of the blank""s movements at the level of the loading wheel is relatively significant. Said vibrations affect the precision of the actual position of the blank""s neck at the level of the loading point so that, in some cases, the gripper device and the neck are offset to the point of preventing the blank from being gripped correctly.
Therefore, the objective of the invention it to suggest means that make is possible to ensure that the preform is perfectly stable when it is gripped by the conveyor chain.
With this end in view, the invention suggests a system of conveyance for blowing installations for thermoplastic containers, of the type that comprises a series of conveyor elements fitted with at least one gripper device used to hold a container blank in order to move it along a path inside the installation, of the type in which each gripper device moves in an axial direction in relation to the conveyor element that carries it so that, when it is brought from a high cleared position to a low gripped position, it engages in an axial direction with the blank""s neck, where the blank then rests by an external radial collar on a bearing surface, characterized by the fact that each conveyor element comprises a stabilizer that can move in an axial direction from a high retracted position to a low position in order to bear upon the blank so that it can be pressed against the bearing surface before the gripper device engages with the neck.
Based on other characteristics of the invention:
the stabilizer""s movements between its high and its low positions arc controlled by the movements of the gripper device;
the gripper device comprises a mandrel that engages in an axial direction inside the blank""s neck; the stabilizer comprises a bore whose diameter is more or less identical to the internal diameter of the neck and in which the mandrel can slide in an axial direction; and, in the high position, the mandrel is received inside the stabilizer whereas, in the low gripping position, the mandrel extends outside the bore, through a lower opening delimited by the stabilizer""s lower bearing edge;
at the beginning of its stroke toward its gripping position, the mandrel drives the stabilizer that comes to rest by its lower edge against the preform in order to press it against the bearing surface; and the mandrel continues its course until it reaches its gripping position;
during its return stroke toward its high cleared position, the mandrel brings the stabilizer back toward its high retracted position;
the mandrel is received and tightened in the stabilizer""s bore in such a way that the mandrel drives the stabilizer by adherence;
the mandrel comprises a ring that is elastic in the radial direction so that, in the mandrel""s high position, it comes to rest against a lateral wall of the stabilizer""s bore and, in the mandrel""s low position, it comes to rest against the inner surface of the blank""s neck;
the gripping device and the stabilizer are connected in an axial direction by elastic means;
the stabilizer slides in an axial direction in a tubular guide sheath that is integral with the conveyor element, and
the stabilizer comes to rest on the upper edge of the blank""s neck;
The invention also relates to a thermal conditioning oven for a blowing installation for thermoplastic containers, characterized by the fact that it comprises a system of conveyance that incorporates any one of the previous characteristics.